This STANDARD lists Track Center Distances and provides for Side Clearances required for various curvatures with three size categories of models.
Class II Includes small four-wheel truck diesels, geared and other small steam locomotives with short end overhangs typical of old-time, logging and branch lines and equivalent rolling stock.
Class I Includes longer steam locomotives typically with two-wheel trailing trucks, larger four and six-wheel truck diesels and equivalent rolling stock.
Class Ia Includes the largest steam locomotives with four-wheel trailing trucks, articulated locomotives, those with rigid wheelbases in excess of
20 feet, full length passenger cars and other long rolling stock.
Layouts constructed to one of these classes should accept models of its own and smaller classes, but larger models can expect clearance problems on a layout built to a smaller classification. See STANDARD S-7 and RP-11.
TRACK CENTERS in CURVES
Curvature in Degrees
M(*)
Radius-Prototype
0
Tang.
5
1146
10
574
15
383
20
288
25
231
30
193
35
166
40
146
45
131
O SCALE:
Radius-inches
Centers
Class II
3
3 -
4
Class I
Class Ia
3
3 -
4
3
3 -
4
On3:
Centers 3
Tang.
1
3 -
4
1
3 -
4
1
3 -
4
3
2 -
4
287
1
3 -
4
3
3 -
8
1
3 -
2
3
2 -
4
144
5
3 --
16
1
3 -
2
13
3 --
16
13
2 --
16
96
3
3 -
8
5
3 -
8
1
4 --
16
15
2 --
16
72
1
3 -
2
3
3 -
4
5
4 --
16
1
3 --
16
58
5
3 -
8
7
3 -
8
9
4 --
16
3
3 --
16
48
3
3 -
4
4
5
3 --
16
1
41 -
2
13
3 --
16
3
4 --
16 note 8
3
3 -
8
1
36 -
2
7
3 -
8 note 8
1
3 -
2
33
4
5
3 -
8
S SCALE:
Radius-inches
Centers
Class II
Class I
Class Ia
Sn3:
Centers
13
2 --
16
13
2 --
16
13
2 --
16
1
2 -
4
Tang.
7
2 --
16
7
2 --
16
7
2 --
16
1
2 --
16
215
7
2 --
16
9
2 --
16
11
2 --
16
1
2 --
16
108
1
2 -
2
5
2 -
8
7
2 -
8
1
2 --
16
72
9
2 --
16
3
2 -
4
1
3 --
16
1
2 -
8
54
5
2 -
8
13
2 --
16
1
3 -
4
1
2 -
4
43
11
2 --
16
7
2 -
8
3
3 -
8
3
2 -
8
36
3
2 -
4
3
7
2 --
16
31
13
2 --
16
1
3 -
8 note 8
9
2 --
16
27
7
2 -
8 note 8
5
2 -
8
25
3
3
2 -
4
OO SCALE:
Radius-inches
Centers
Class I
Class Ia
Tang.
1
180 -
2
1
90 -
2
60
1
45 -
2
1
36 -
2
1
30 -
2
26 23
1 1 1 1 1 1
2 - 2 - 2 - 2 - 2 2 -
5
2 --
1
2 -
3
2 - note 8
2 4 4 4 4 4 16 2 4
1 1 1 1 7 11 15 3
2 - 2 - 2 - 2 - 2 -- 2 -2 -- 3 --
7
3 -- note 8
2 4 4 4 16 16 16 16 16
1
20 -
2
(*)M = 2A (from S-7) = Preferred Minimum Track Center Distance for easy handling, coupling, yards, etc.
S-8 Track Centers - 02/25/98 - p1

Degrees
Radius
HO SCALE:
Radius-inches
M
Centers
Class II
Class I
Class Ia
HOn3:
Centers
1
2 --
16
1
2 --
16
1
2 --
16
11
1 --
16
TT SCALE:
Radius-inches
Centers
Class II
Class I
Class Ia
TTn3:
Centers
1
1 -
2
1
1 -
2
1
1 -
2
1
1 -
4
N SCALE:
Radius-inches
Centers
Class II
Class I
Class Ia
3
1 --
16
3
1 --
16
3
1 --
16
1
1 --
32
1
1 --
32
1
1 --
32
Tang.
86
1
1 --
32
1
1 --
16
1
1 -
8
43
1
1 --
32
3
1 --
32
7
1 --
32
0
Tang.
5
1146
10
574
Tang.
158 79
13
1 --
16
13 l --
16
13 l --
16
9
1 --
16
13 l --
16
7
1 -
8
31
1 --
32
9
1 --
16
5
1 --
16
5 l --
16
5 l --
16
1
1 -
8
Tang.
1
114 -
2
11 l --
32
3
1 -
8
7
1 --
16
5
1 --
32
1
57 -
2
3 l -
8
7
1 --
16
9
1 --
16
3
1 --
16
13 l --
16
15
1 --
16
1
2 -
8
9
1 --
16
29
1
1 --
16
1
1 -
8
9
1 --
32
29
7
1 --
16
17
1 --
32
3
1 -
4
1
1 -
4
1
21 -
2
3
1 --
32
5
1 --
32
11
1 --
32
15
383
53
7
1 -
8
2
1
2 -
4
5
1 -
8
1
38 -
2
13
1 --
32
1
1 -
2
11
1 --
16
7
1 --
32
20
288
40
15
1 --
16
1
2 --
16
3
2 -
8
11
1 --
16
25
231
32
17
1
1 -
8
7
1 --
32
13
1 --
32
2
1
2 -
8
1
2 -
2
3
1 -
4
23
15
1 --
32
9
1 --
16
13
1 --
16
9
1 --
32
13
1 --
16
1
19 -
2
1
1 -
2
19
1 --
32
5
1 --
16
1
14 -
2
5
1 --
32
1
1 -
4
30
193
1
26 -
2
1
2 --
16
3
2 --
16
35
166
40
146
45
131
23 20 18
3
2 --
32
5
2 --
16 note 8
1
2 -
8 note 8
3
2 --
16
7
1 -
8
1
16 -
2
17
1 --
32
5
1 -
8 note 8
15
1 --
16
1
14 -
2
9
1 --
16 note 8
2
13
19
1 --
32
11
1 --
32
1
12 -
2
3
1 --
16
5
1 --
16 note 8
3
1 -
8
11
13
1 --
32
10
7
1 --
32 note 8
1
1 -
4
NOTES:
1. Radius is measured at track centerline - midway between rails - of the inner of multiple tracks.
2. For a radius intermediate of those listed use the next larger radius shown in the table.
3. Dual gage track shall use standard gage radii, centers and clearances.
4. Narrow gage locomotives and other equipment built on standard gage frames, such as K types, require standard gage Track
Centers.
5. Increased Track Centers and clearances for curves shall be eased (see D3 series DATA SHEETS) for a distance along the track
consistent with the class equipment used.
6. Inside Clearance shall be half the Track Center Distance listed up to, but not to exceed, M/2 (A from S-7).
7. Outside Clearance shall be half the Track Center Distance listed up to M/2 (A from S-7); for Track Centers Greater than M,
Outside Clearance shall be M/2 plus the amount Track Center Distance exceeds M (2A from S-7).
8. Construction or operation of equipment on curves sharper than those listed for its class are neither prohibited nor recommended.
S-8 Track Centers - 02/25/98 - p2